Physical activity (PA) decreases by an alarming 50 percent during the second decade of life. Despite the decrease in this important health behavior, little is known about the role of physical activity cognitions in shaping positive or negative responses of adolescents to singular episodes of physical activity. The overall purpose of the proposed study is to determine if physical activity cognitions (self- efficacy, self-schema) and prior enjoyment of activity modify perceived exertion and affective responses of adolescents during a challenging episode of physical activity. The study will also explore if physical activity cognitions and responses to activity differ by stage of maturation, gender or race. A 3(level of maturation) x 2 (gender) x 2 (race) research design with repeated measures will be used. Bases on power analysis, a total of 144 subjects, 12 per demographic category, will be required for the study. Subjects will complete two testing sessions. In the first session, peak VO2, adiposity, and cognitive measures will be determined. At a second session, subjects will walk/run on a treadmill at 60 percent of peak VO2 for 20 minutes, reporting perceived exertion and affect at 4- minute intervals. Post-episode measures of enjoyment and self-efficacy will be administered. Research findings will contribute new knowledge concerning the link between physical activity cognitions and responses during physical activity among adolescents of differing maturation, gender and race. This information will enhance understanding of how to increase positive responses to PA and how to design nursing interventions to promote long-term participation in PA among diverse groups of youth.